Needle bearing and method for grinding bearing parts thereof

ABSTRACT

It is an object of the invention to provide a needle bearing which can prevent occurrence of flake and seizure in the bearing parts even under such severe lubricating condition as provided by a compressor for a car air conditioner or a planetary unit for an automatic transmission. The needle bearing is structured by barrel grinding the raceway surface of a ring and the surface of a rolling body without using a ceramic-related abrasive such as alumina or silicon carbide.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a needle bearing which is usedin a car air conditioner or in a planetary unit for an automatictransmission.

[0002] In a compressor for a car air conditioner or in a planetary unitfor an automatic transmission, as a bearing for supporting a thrust loadapplied to a rotary part, there is used a thrust needle bearing.However, in the compressor for a car air conditioner, since HFC134a(Hydro Fluoro Carbon 134a) or PAG (Polyalkylene glycol) is used as arefrigerant or operating oil, the lubricating condition of the bearingis under a very severe environment. And, the rotation condition of thebearing also ranges widely from high speeds to low speeds, and furtherthe load to be applied to the bearing ranges from a condition of almostno load to a condition of about 5000N; and, depending on the operatingconditions of the compressor, these conditions are combined together atrandom.

[0003] Therefore, under the environment requiring such severelubricating condition, there is required a thrust needle bearing whichcan provide better lubricating performance and, to satisfy suchrequirement, conventionally, for example, the surface roughness of aring or a rolling body is enhanced or, as shown in FIGS. 19A and 19B, ina thrust ring 1, there are formed notches 2 which are used to secure anoil passage. Also, as shown in FIG. 20, in order to secure the passingoil amount of lubricating oil, the collar portions 4 of an outer ring 3are turned over to thereby increase the opening height h.

[0004] However, in the compressor for a car air conditioner or in theplanetary unit for an automatic transmission (A/T), originally, sincethe lubricating condition is very severe, the above-mentioned measurescan provide little effect. Also, because the lubricating condition isvery severe, for example, there occurs metal contact between the ringand rolling body. Therefore, in case where foreign substances higher inhardness than the rolling body are sticking to the raceway surface ofthe ring and the pocket inner surface of the retainer to be contactedwith the rolling body, the rolling body is worn due to the influence ofthe foreign substances, thereby raising a fear that the bearing can beflaked or seized.

SUMMARY OF THE INVENTION

[0005] The present invention aims at eliminating the above-mentioneddrawbacks found in the conventional needle bearing. Accordingly, it isan object of the invention to provide a needle bearing which can preventthe occurrence of the flaking and seizure even under such severelubricating condition environment as provided by a compressor for a carair conditioner or a planetary unit for an automatic transmission.

[0006] In attaining the above object, according to a first aspect of theinvention, there is provided a needle bearing comprising: a retainerhaving a plurality of pockets; rings; and a plurality of rolling bodiesrollably incorporated in the respective pockets of the retainer androlling on raceway surfaces of the rings, wherein at least one of theretainer, the rolling bodies and the rings is free from a foreignsubstance having hardness equal to or higher than the hardness of thebase material of the rolling bodies.

[0007] According to a second aspect of the invention, there is provideda method for grinding bearing parts, wherein, when grinding surfaces ofthe bearing parts made of metal using abrasives, the abrasives havehardness lower than hardness of base material of the bearing parts, andthe surfaces of the bearing parts are grinded using the abrasives by atleast one of barrel grinding, shot blasting, and shot peening.

[0008] Also, according to a third aspect of the invention, there isprovided a needle bearing comprising: a retainer having a plurality ofpockets; rings; and a plurality of rolling bodies rollably incorporatedin the respective pockets of the retainer and rolling on racewaysurfaces of the rings, wherein at least one of the retainer, the rollingbodies and the rings is free from, on the surface thereof, a foreignsubstance having a mean diameter of larger than 30 μm.

[0009] In the first to third aspects of the invention, as specific meansfor preventing a foreign substance having hardness equal to or higherthan the hardness of the base material of the rolling bodies or aforeign substance having a mean diameter larger than 30 μm from beingpresent on the surface of the ring, there can be employed a method forbarrel working the raceway surface without using a ceramic-relatedabrasive such as alumina (hardness Hv of 1800 to 2200) or siliconcarbide (hardness Hv of 2500 to 3000), that is, a method for barrelworking the raceway surface using an abrasive lower in hardness than thebase material of the rolling bodies and ring of the bearing. And, in thecase of the method using a ceramic-related abrasive, as a post-workingstep, there is used a barrel polishing method for polishing the needlebearing using plastic material, wall nut material and corn material(coat of corn), or a method for removing the ceramic-related abrasivethrough a washing operation. By the way, the mean diameter of theforeign substance here means the mean of the major minor diameters ofone particle.

[0010] According to a fourth aspect of the invention, there is provideda needle bearing comprising: a retainer having a plurality of pockets;rings; and a plurality of rolling bodies rollably incorporated in therespective pockets of the retainer and rolling on raceway surfaces ofthe rings, wherein, in case where 0.01 mm² of the surface of at leastone of the bearing parts, that is, the retainer, the rolling bodies andthe ring is set as a unit area, the rate per unit area of existence ofhard foreign substances having hardness equal to or higher than thehardness of the base material of the rolling bodies and sticking to thesurfaces of the bearing parts is set equal to or less than 10%.According to a fifth aspect of the invention, there is provided a needlebearing comprising: a retainer having a plurality of pockets; rings; anda plurality of rolling bodies rollably incorporated in the respectivepockets of the retainer and rolling on raceway surfaces of the rings,wherein, in case where 0.01 mm² of the surface of at least one of thebearing parts, that is, the retainer, the rolling bodies and the ring isset as a unit area, the number per unit area of hard foreign substanceshaving hardness equal to or higher than the hardness of the basematerial of the rolling bodies and sticking to the surfaces of thebearing parts is set equal to or less than 300.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a section view of the structure of a needle bearingaccording to an embodiment of the invention;

[0012]FIG. 2 is a graphical representation of the bus shape of thesurface of a rolling body obtained when glass beads are used asabrasives;

[0013]FIG. 3 is a graphical representation of the bus shape of thesurface of a rolling body obtained when alumina oxide is used asabrasives;

[0014]FIG. 4 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 1, shown in Table 1 before and after a wear test isconducted;

[0015]FIG. 5 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 4, shown in Table 1 before and after a wear test isconducted;

[0016]FIG. 6 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 5, shown in Table 1 before and after a wear test isconducted;

[0017]FIG. 7 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 9, shown in Table 1 before and after a wear test isconducted;

[0018]FIG. 8 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 8, shown in Table 1 before and after a wear test isconducted;

[0019]FIG. 9 shows the shape of a bus of a rolling body used in a samplebearing, sample No. 11, shown in Table 2 before and after a wear test isconducted;

[0020]FIG. 10 shows the shape of a bus of a rolling body used in asample bearing, sample No. 13, shown in Table 2 before and after a weartest is conducted;

[0021]FIG. 11 shows the shape of a bus of a rolling body used in asample bearing, sample No. 18, shown in Table 2 before and after a weartest is conducted;

[0022]FIG. 12 shows the shape of a bus of a rolling body used in asample bearing, sample No. 22, shown in Table 2 before and after a weartest is conducted;

[0023]FIG. 13 shows the shape of a bus of a rolling body used in asample bearing, sample No. 15, shown in Table 2 before and after a weartest is conducted;

[0024]FIG. 14 shows the shape of a bus of a rolling body used in asample bearing, sample No. 31, shown in Table 3 before and after a weartest is conducted;

[0025]FIG. 15 shows the shape of a bus of a rolling body used in asample bearing, sample No. 32, shown in Table 3 before and after a weartest is conducted;

[0026]FIG. 16 shows the shape of a bus of a rolling body used in asample bearing, sample No. 37, shown in Table 3 before and after a weartest is conducted;

[0027]FIG. 17 shows the shape of a bus of a rolling body used in asample bearing, sample No. 42, shown in Table 3 before and after a weartest is conducted;

[0028]FIG. 18 shows the shape of a bus of a rolling body used in asample bearing, sample No. 39, shown in Table 3 before and after a weartest is conducted;

[0029]FIGS. 19A and 19B are a perspective view of a thrust ring used ina conventional thrust needle bearing; and,

[0030]FIG. 20 is a section view of the structure of a conventionalradial needle bearing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Now, description will be given below of the preferred embodimentsof a needle bearing according to the invention with reference to theaccompanying drawings.

[0032]FIG. 1 is a section view of the structure of a needle bearingaccording to an embodiment of the invention. As shown in FIG. 1, aneedle bearing according to the present embodiment of the inventioncomprises a ring 11, a plurality of rolling bodies (needle-like rollers)12, and a retainer 13.

[0033] The ring (hardness Hv of 650 to 800) 11, rolling bodies (hardnessHv of 700 to 800) 12 and retainer (hardness Hv of 650 to 800) 13 arerespectively made of metal such as SUJ2. In the retainer 13, there areopened up a plurality of pockets (not shown) for holding the rollingbodies 12 in a rollable manner, while the pockets are disposed atregular intervals in the circumferential direction of the ring 11. Also,the rolling bodies 12 as well as the raceway surface 11 a of the ring 11and the pocket inner surfaces 13 a of the retainer 13 to be contactedwith the rolling bodies 12 are worked by barrel grinding using anabrasive (such as a glass bead (hardness Hv of 400 to 500) and a metalball (hardness Hv of 400 to 600)) lower in hardness than the basematerial of bearing parts such as the rolling bodies 12 and ring 11 inorder to remove scales and burrs produced when the present needlebearing is heat treated.

[0034] When barrel grinding the bearing parts of the thrust needlebearing, in case where the surfaces of the bearing parts are groundusing an abrasive lower in hardness than the base material of thebearing parts, there can be obtained the following effect. That is, evenincase where the abrasives are not removed perfectly in a washing stepafter a grinding operation but the abrasives are left on the surfaces ofthe bearing parts, when the thrust needle bearing is in actual use, theremaining abrasives are bitten into and crushed in, for example, betweenthe thrust ring 11 and rolling bodies 12. Therefore, even in case wherethe abrasives are left on the surfaces of the bearing parts, they existno longer as foreign substances which are higher in hardness than thebase material of the bearing parts, thereby being able to reduce theinfluence of the abrasives remaining on the surfaces of the bearingparts.

[0035] To confirm the above effect, the present inventors, using athrust needle bearing having an inside diameter of 40 mm, an outsidediameter of 60 mm and a thickness of 5 mm (rated dynamic load: 24000N)as a test bearing, have conducted a rolling body wear resistance testunder the following test conditions: that is, the axial load: 2000N, theradial load: ON, the rotation speed: 2000 min⁻¹, lubricating oil:kerosene (PAG=9:1), and test time: 48 hrs. And, after completion of thetest, the present inventors have observed the rolling body surfaces ofthe respective test bearings. FIGS. 2 and 3 show the results of ourobservation.

[0036] Specifically, FIG. 2 shows the bus shape of the rolling bodysurface when glass beads (#120, diameter of the order of 0.1 to 0.2 mm)were used as the abrasives, whereas FIG. 3 shows the bus shape of therolling body when aluminum oxide was used as the abrasives.

[0037] As can be seen from the observation results shown in FIGS. 2 and3, when aluminum oxide was used as the abrasives, in 48 hrs. after startof the test, the rolling body 12 wore down to a position shown by asolid line in FIG. 3. On the other hand, in case where glass beads wereused as the abrasives, when compared with the case using aluminum oxideas the abrasives, the wear of the rolling body 12 was improved greatly.

[0038] The above observation results show the following facts. That is,when barrel grinding bearing parts made of metal, preferably, thesurfaces of the bearing parts may be ground using abrasives having lowerhardness than the base material of the bearing parts. Specifically,because the hardness of the abrasives is lower than that of the basematerial of the bearing parts, even in case where the abrasives are notremoved perfectly in a washing step after a grinding operation but theabrasives are left on the surfaces of the bearing parts, when the thrustneedle bearing is in actual use, the remaining abrasives are bitten intoand crushed in, for example, between the thrust ring 11 and rolling body12. Therefore, even in case where the abrasives are left on the surfacesof the bearing parts, they exist no longer as foreign substances whichare higher in hardness than the base material of the bearing parts,thereby being able to reduce the influence of the abrasives remaining onthe surfaces of the bearing parts.

[0039] By the way, the invention is not limited to the above-mentionedembodiment. For example, in the above embodiment, as the abrasives forgrinding the surfaces of the bearing parts, there are used glass beads;however, as the abrasives, there can also be used a metal ball (diameterof about 1 mm) lower in hardness than the base material of the bearingparts. Also, in the above embodiment, the surfaces of the bearing partsare worked by barrel grinding. However, they may also be worked by shotblasting or by shot peening.

[0040] By the way, the inventors observed the bearing as to the wearconditions generated therein and found, in the raceway surface androlling bodies, not only bitten traces caused by foreign substances suchas worn metal powder but also worn states caused by microscopic foreignsubstances. When observing these bitten traces and worn states in moredetail, it has been confirmed that the same bitten traces and wornstates are repeatedly left in the raceway surface and rolling bodies ofthe bearing.

[0041] The above observation results suggest that the bearing is worndue to mixture of the foreign substances into the raceway surface androlling bodies: that is, it is assumed that the foreign substances arefixed to the raceway surface and rolling body (the foreign substancesare buried into one of the raceway surface and rolling body which islower in hardness), the thus buried foreign substances repeatedly damagethe rolling body to thereby cause the rolling body to wear.

[0042] Thus, the present inventors also observed the raceway surface androlling body of the bearing as to their initial states before they areworn due to the foreign substances. According to the results of thisobservation, it has been confirmed that, in the initial states as well,there are repeatedly caused similar bitten traces to the above case.Further, on the raceway surface of the bearing in the initial state, thepresence of the foreign substance was also confirmed and, when theforeign substance was checked for the component thereof, the componentwas found a ceramic component such as aluminum oxide which is anon-metal component.

[0043] Further, when the raceway surface of a bearing, which had notbeen used, was observed, such ceramic-related foreign substance asmentioned above, such as alumina, was found. This shows a possibilitythat the foreign substance has been already stuck to the bearing whenassembly of the bearing is completed. It is believed that a media (anabrasive) and a compound used when the bearing is worked by barrelgrinding are left on the raceway surface of the bearing to provide suchforeign substance.

[0044] The above observations and assumptions show a possibility that asmall amount of media remaining on the raceway surface of the bearingcan cause the bearing to wear and thus can shorten the life of thebearing greatly.

[0045] To confirm the above possibility, the present inventors, using athrust needle bearing having an inside diameter of 40 mm, an outsidediameter of 60 mm and a thickness of 5 mm (rated dynamic load: 24000N)as a test bearing, have conducted a rolling body wear resistance testunder the following test conditions: that is, the axial load: 2000N, theradial load: ON, the rotation speed: 2000 min⁻¹, lubricating oil:kerosene (PAG=9:1), and test time: 48 hrs. And, the number of foreignsubstances per unit area existing on the bearing raceway surface and themaximum diameter of the foreign substances were confirmed through anelectronic microscope before start of the test; and, after completion ofthe test, the rolling body wear amounts of the respective test bearingswere measured. Table 1 shows the results of the measurement. TABLE 1Maximum diameter Bearing wear Sample Abrasive used in Number of offoreign substance Amounts Nos. barrel grinding foreign substances (μ m)(μ m) 1 No abrasive 3 5 0 2 No abrasive 5 11 0 3 No abrasive 9 32 0 4 Noabrasive 5 43 10 5 Alumina 7 7 0 6 Alumina 18 26 0 7 Alumina 20 32 1 8Alumina 17 52 10 9 Silicon carbide 14 33 1 10 Silicon carbide 23 53 15

[0046] In Table 1, sample numbers 1 to 4 show sample bearings which areassembled using rings on which barrel working (grinding operation) isnot enforced, whereas sample numbers 5 to 8 show sample bearings onwhich alumina is used as a media or as an abrasive when they are workedby barrel grinding. Also, sample numbers 9 and 10 show sample bearingson which silicon carbide is used as an abrasive when they are worked bybarrel grinding. By the way, in Table 1, the maximum diameter of aforeign substance means the greatest one of the means of the majordiameter and minor diameter of one particle.

[0047] Also, the present inventors have observed the surfaces of therolling bodies of the respective sample bearings after completion of theabove test. FIGS. 4 to 8 show the results of this observation. Here,FIG. 4 shows the bus shape of the rolling body in the sample bearingwhich is designated by the sample No. 1, FIG. 5 shows the bus shape ofthe rolling body in the sample bearing designated by the sample No. 4,FIG. 6 shows the bus shape of the rolling body in the sample bearingdesignated by the sample No. 5, FIG. 7 shows the bus shape of therolling body in the sample bearing designated by the sample No. 9, andFIG. 8 shows the bus shape of the rolling body in the sample bearingdesignated by the sample No. 8, respectively. Also, in each of thesefigures, a broken line shows the bus shape of the rolling body beforestart of the test, whereas a solid line shows the bus shape of therolling body after completion of the test.

[0048] From the bus shapes of the rolling bodies shown in FIGS. 4 to 8,there can be confirmed the following facts: that is, in the case of thesample bearing No. 1, since no abnormal condition is found in the shapeof the rolling body, it is confirmed that the sample bearing No. 1 hasnot been worn. Also, in the case of the sample bearing No. 4, the shapeof the bus of the rolling body of the sample bearing provides such shapeas shown by a broken line before the test, whereas it provides suchshape as shown by a solid line after the test; and, therefore, it isconfirmed that the sample No. 4 has been worn. Further, in the case ofthe sample bearings Nos. 5 and 9, the media was removed from them by awashing treatment after completion of barrel grinding and thus theceramic-related media having a mean diameter of 30 μm was not found onthe raceway surfaces of these sample bearings; and therefore, it isconfirmed that the rolling bodies have not been worn by foreignsubstances. In addition, in the case of the sample bearing No. 8, it isconfirmed that the rolling body thereof has been worn up to such aposition as shown by a solid line due to the ceramic-related mediaremaining on the raceway surface of the sample bearing.

[0049] From the above test, we can reach the following conclusion. Thatis, in case where a needle bearing is structured such that at least oneof the ring, rolling body and retainer of the needle bearing does notinclude on the surface thereof a foreign substance equal to or higher inhardness than the base material of the needle bearing, or a substancehaving a mean diameter larger 30 μm, the ring and rolling body can beprevented from wearing. Therefore, even under such severe lubricatingconditions as provided by a compressor for a car air conditioner and aplanetary unit for automatic transmission, the bearing parts can beprevented from being flaked or seized.

[0050] Next, the present inventors, in order to check the remainingabrasives for the influence thereof, conducted a bearing wear test underthe following test conditions I employing not only needle bearings whichhad been worked by barrel grinding using an alumina-related abrasive(media) but also needle bearings worked by barrel grinding using asilicon-carbide-related abrasive. And, before the test, the rate perunit area (which is 0.01 mm² of the ring way surface of a bearing) ofthe remaining abrasive was measured; and, after the test, the wearamounts of the rolling bodies of the respective needle bearings weremeasured. Table 2 shows the results of these measurements.

[0051] Test Conditions I

[0052] (1) Sample bearing: Thrust needle bearing having: Insidediameter: 40 mm, Outside diameter: 60 mm, Thickness: 5 mm, Rated dynamicload: 24000N,

[0053] (2) Axial load: 3000N

[0054] (3) Radial load: ON

[0055] (4) Rotation speed: 1800 min⁻¹

[0056] (5) Lubricating oil: Kerosene (PAG=9:1)

[0057] (6) Test time: 48 hrs. TABLE 2 Sample Abrasive used in Areaoccupied by Bearing wear Nos. barrel grinding foreign substance (%)Amounts (μ m) Judgment Figure 11 No abrasive 3% or less 0 Good 7 12 Noabrasive 7 2 Good — 13 No abrasive 21 7 No good 8 14 No abrasive 13 3Good — 15 Alumina 22 15 No good 11 16 Alumina 5 5 Good — 17 Alumina 7 4Good — 18 Alumina 12 1 Good 9 19 Alumina 9 0 Good — 20 Silicon carbide13 5 Good — 21 Silicon carbide 4 3 Good — 22 Silicon carbide 11 1 Good10

[0058] In Table 2, sample Nos. 11 to 13 designate thrust needle bearingsin which, after they were worked by barrel grinding, the racewaysurfaces of the bearings were washed and the media was removedtherefrom; sample Nos. 15 to 19 designate thrust needle bearings inwhich, as an abrasive in barrel working, there was used alumina; and,sample Nos. 20 to 22 designate thrust needle bearings in which, as anabrasive in barrel working, there was used silicon carbide.

[0059] As can be clearly understood from the measurement results shownin Table 2, the thrust needle bearings designated by the sample Nos. 11,12, 14, 16 to 22 are smaller in the bearing wear amount than the thrustneedle bearings designated by the sample Nos. 13 and 15. The reason forthis is believed that, in the case of the thrust needle bearings, sampleNos. 13 and 15, the rate per unit area of the remaining abrasive exceeds10%, whereas in the case of the thrust needle bearings, sample Nos. 11,12, 14, 16 to 22, the rate per unit area of the remaining abrasive isequal to or less than 10%.

[0060] Also, the inventors observed the rolling body bus shapes of therespective bearings after completion of the above test. FIGS. 9 to 13show the results of this observation. Here, FIG. 9 shows the rollingbody bus shape of the bearing, sample No. 11; FIG. 10 shows the rollingbody bus shape of the bearing, sample No. 13; FIG. 11 shows the rollingbody bus shape of the bearing, sample No. 18; FIG. 12 shows the rollingbody bus shape of the bearing, sample No. 22; FIG. 13 shows the rollingbody bus shape of the bearing, sample No. 15, respectively. Also, ineach of these figures, a broken line shows the bus shape of the rollingbody before the test was started, whereas a solid line shows the busshape of the rolling body before the test was ended.

[0061] As can also be clearly seen from the rolling body bus shapesshown in FIGS. 9 to 13, in the case of the needle bearings, sample Nos.11, 18, 22, no abnormal condition is found in the bus shapes of therolling bodies thereof and thus it is confirmed that the rolling bodieshave been little worn. On the other hand, in the case of the needlebearings, sample Nos. 13 and 15, an abnormal condition is found in thebus shapes of the rolling bodies thereof and thus it is confirmed thatthe rolling bodies have been worn.

[0062] From the above test results, the inventors can reach thefollowing conclusion. That is, when 0.01 mm² of the raceway surface isused as a unit area, in case where the rate per unit area of a hardforeign substance such as the remaining abrasive is set equal to or lessthan 10%, the amount of wear of the bearing parts due to the foreignsubstance such as the remaining abrasive can be reduced, which makes itpossible to prevent occurrence of flaking and seizure in the bearingparts.

[0063] Next, the inventors, in order to check the remaining abrasive forthe influence thereof, conducted a bearing wear test under the followingtest conditions II using not only needle bearings which had beenrespectively worked by barrel grinding with an alumina-related abrasive(media) but also needle bearings respectively worked by barrel grindingwith a silicon-carbide-related abrasive. And, before the test, thenumber per unit area (which is 0.01 mm² of the raceway surface of abearing) of the remaining abrasive was measured; and, after the test,the wear amounts of the rolling bodies of the respective needle bearingswere measured. Table 3 shows the results of these measurements.

[0064] Test Conditions II

[0065] (1) Sample bearing: Thrust needle bearing having: Insidediameter: 40 mm, Outside diameter: 60 mm, Thickness: 5 mm, Rated dynamicload: 24000N,

[0066] (2) Axial load: 2000N

[0067] (3) Radial load: ON

[0068] (4) Rotation speed: 2000 min⁻¹

[0069] (5) Lubricating oil: Kerosene (PAG=9:1)

[0070] (6) Test time: 48 hrs. TABLE 3 Sample Abrasive used in Number offoreign Bearing wear Nos. barrel grinding substances Amounts (μ m)Judgment Figure 31 No abrasive 3 0 Good 12 32 No abrasive 280 2 Good 1333 No abrasive 340 6 No good — 34 No abrasive 320 3 Good — 35 Noabrasive 300 3 Good — 36 Alumina 320 2 Good — 37 Alumina 285 3 Good 1438 Alumina 135 1 Good — 39 Alumina 390 10 No good 16 40 Alumina 320 7 Nogood — 41 Silicon carbide 250 4 Good — 42 Silicon carbide 280 4 Good 1543 Silicon carbide 330 5 Good —

[0071] In Table 3, sample Nos. 31 to 35 designate thrust needle bearingsin which, after they were worked by barrel grinding, the racewaysurfaces of the bearings were washed and the media was removedtherefrom; sample Nos. 36 to 40 designate thrust needle bearings inwhich, as an abrasive in barrel working, there was used alumina; and,sample Nos. 41 to 43 designate thrust needle bearings in which, as anabrasive in barrel working, there was used silicon carbide.

[0072] As can be clearly seen from the measurement results shown inTable 3, the thrust needle bearings respectively designated by thesample Nos. 31, 32, 34 to 38, and 41 to 43 are smaller in the bearingwear amount than the thrust needle bearings respectively designated bythe sample Nos. 33, 39, and 40. The reason for this is believed that, inthe case of the thrust needle bearings, sample Nos. 33, 39, and 40, thenumber per unit area of the remaining abrasives exceeds 300, whereas inthe case of the thrust needle bearings, sample Nos. 31, 32, 34 to 38,and 41 to 43, the number per unit area of the remaining abrasives isequal to or less than 300.

[0073] Also, the inventors observed the rolling body bus shapes of therespective bearings after completion of the above test. FIGS. 14 to 18show the results of this observation. Here, FIG. 14 shows the rollingbody bus shape of the bearing, sample No. 31; FIG. 15 shows the rollingbody bus shape of the bearing, sample No. 32; FIG. 16 shows the rollingbody bus shape of the bearing, sample No. 37; FIG. 17 shows the rollingbody bus shape of the bearing, sample No. 42; and, FIG. 18 shows therolling body bus shape of the bearing, sample No. 39, respectively.Also, in each of these figures, a broken line shows the bus shape of therolling body before the test was started, whereas a solid line shows thebus shape of the rolling body before the test was ended.

[0074] As can also be clearly seen from the rolling body bus shapesshown in FIGS. 14 to 18, in the case of the needle bearings, sample Nos.31, 32, 37 and 42, no abnormal condition is found in the bus shapes ofthe rolling bodies thereof and thus it is confirmed that the rollingbodies have been little worn. On the other hand, in the case of theneedle bearing, sample No. 39, an abnormal condition is found in the busshape of the rolling body thereof and thus it is confirmed that therolling body has been worn.

[0075] From the above test results, the inventors can reach thefollowing conclusion. That is, when 0.01 mm² of the raceway surface isused as a unit area, in case where the number per unit area of hardforeign substances such as the remaining abrasives is set equal to orless than 300, the amount of wear of the bearing parts due to theforeign substances such as the remaining abrasives can be reduced, whichmakes it possible to prevent occurrence of flaking and seizure in thebearing parts.

[0076] By the way, the invention is not limited to the above illustratedembodiments. For example, in the above illustrated embodiments, theinvention is applied to a thrust needle bearing but, of course, theinvention can also be applied to a radial needle bearing. In the case ofthe radial needle bearing, there can also be employed specifications inwhich a cage and rollers (a retainer and rolling bodies) are employed,and the rolling bodies thereof are all composed of rollers.

[0077] As has been described heretofore, according to the first to thirdaspects of the invention, since foreign substances higher in hardnessthan the base material of rolling bodies exist no longer on the surfaceof the ring, the surface of the rolling body and the inner surface ofthe retainer, even under such severe lubricating conditions as providedby a compressor for a car air conditioner and a planetary unit forautomatic transmission, the bearing parts can be prevented from beingflaked or seized.

[0078] And, according to the fourth and fifth aspects of the invention,because the wear amounts of the bearing parts due to the foreignsubstances such as the remaining abrasives can be reduced, the bearingparts can be prevented from being flaked or seized.

What is claimed is:
 1. A needle bearing comprising: a retainer having aplurality of pockets; rings; and a plurality of rolling bodies rollablyincorporated in the respective pockets of the retainer and rolling onraceway surfaces of the rings, wherein at least one of the retainer, therolling bodies and the rings is free from a foreign substance havinghardness equal to or higher than the hardness of the base material ofthe rolling bodies.
 2. A method for grinding bearing parts, wherein,when grinding surfaces of the bearing parts made of metal usingabrasives, the abrasives have hardness lower than hardness of basematerial of the bearing parts, and the surfaces of the bearing parts aregrinded using the abrasives by at least one of barrel grinding, shotblasting, and shot peening.
 3. A needle bearing comprising: a retainerhaving a plurality of pockets; rings; and a plurality of rolling bodiesrollably incorporated in the respective pockets of the retainer androlling on raceway surfaces of the rings, wherein at least one of theretainer, the rolling bodies and the rings is free from, on the surfacethereof, a foreign substance having a mean diameter of larger than 30μm.
 4. A needle bearing comprising: a retainer having a plurality ofpockets; rings; and a plurality of rolling bodies rollably incorporatedin the respective pockets of the retainer and rolling on racewaysurfaces of the rings, wherein, in case where 0.01 mm² of the surface ofat least one of the bearing parts, that is, the retainer, the rollingbodies and the ring is set as a unit area, the rate per unit area ofexistence of hard foreign substances having hardness equal to or higherthan the hardness of the base material of the rolling bodies andsticking to the surfaces of the bearing parts is set equal to or lessthan 10%.
 5. A needle bearing comprising: a retainer having a pluralityof pockets; rings; and a plurality of rolling bodies rollablyincorporated in the respective pockets of the retainer and rolling onraceway surfaces of the rings, wherein, in case where 0.01 mm² of thesurface of at least one of the bearing parts, that is, the retainer, therolling bodies and the ring is set as a unit area, the number per unitarea of hard foreign substances having hardness equal to or higher thanthe hardness of the base material of the rolling bodies and sticking tothe surfaces of the bearing parts is set equal to or less than 300.